In general, processes for fluidized bed roasting of sulfide materials are technically and commercially known. The raw materials (ore concentrates) are fed into a roaster where they are fluidized, in a fluidized bed which is maintained by passing air upwards through a grate or distributor plate which incorporates numerous air nozzles that pass through the grate or plate. The fluidizing gas, typically air, contains oxygen which reacts with the sulfidic material to convert sulfides to oxides. The depth of the fluidized bed is controlled by withdrawing roasted concentrate either as bed overflow or underflow. The gas, after passing through the bed and fluidizing the bed, may contain finer particles that are entrained in the gas flow and are subsequently separated from the gas by known techniques such as filters or electrostatic precipitators. The gas from the bed contains sulfur dioxide, so the gas is typically sent to a sulfuric acid plant. The roasted product is generally referred to as calcine. The oxidation of sulfidic compounds in the material is auto-thermal and excess heat is available from the oxidation reaction. Examples of sulfidic minerals processed in fluidized bed roasters include materials that contain sulfides of zinc, copper, lead, iron, nickel and molybdenum.
It can be desirable to provide that the amount of oxygen that is available for interaction with the sulfidic material is different at different locations on the grate or distributor plate, for instance to be able to handle different characteristics of the bed material nearer to, and farther from, the zone into which the sulfidic material is fed.
Previous techniques for varying the amount of oxygen that is passed into the bed of sulfidic material have generally varied the number of passages, and/or varied the size of the passages, through the grate or distributor plate, through which the oxygen-containing gas is fed into the bed from the space below the bed. Thus, where it is desired to provide a higher oxygen content in one region of the bed, one would provide more passages and/or would provide larger passages, relative to the number of passages and/or the size of the passages that feed oxygen-containing fluidizing gas to other regions of the bed.
This technique is described in U.S. Pat. No. 7,044,996, which teaches that an oxygen deficit in the vicinity of the area (the feed grate) where bed material is fed into the roaster can be remedied by increasing the number of gas nozzles in the vicinity of the feed grate, and by using bigger gas nozzles in the vicinity of the feed grate, relative to the number of gas nozzles and the size of gas nozzles that are used to feed gas at the rest of the grate. This patent refers to these as techniques to increase the “oxygen content” of the gas that is fed at the feed grate, but it is clear that this patent means by “oxygen content” the total overall amount of oxygen that is fed to one region or another on the grate. U.S. Pat. No. 7,044,996 does not at all recognize the innovation which the present inventors have discovered that relates to increasing the actual oxygen concentration of the fluidizing gas which passes through a selected limited number of the passages through the grate, relative to the actual oxygen concentration of the fluidizing gas that passes through other passages into other regions of the bed. In particular, in comparing the present invention to the disclosure of U.S. Pat. No. 7,044,996, it can be seen that U.S. Pat. No. 7,044,996 does not contain any disclosure of providing such varied oxygen concentrations passing through various openings in the grate, nor any disclosure of how one might accomplish providing such varied oxygen concentrations. Instead, U.S. Pat. No. 7,044,996 teaches only the use of only a single gaseous oxygen-containing fluidizing gas passing through every one of the passages in the grate.